For large or heavy vehicles, it is known to provide brake actuators with two brake mechanisms, namely a service brake for deceleration when the vehicle is in motion, and a parking brake to hold the vehicle stationery.
Conventionally such brake actuators include a first pressure responsive element, such as a diaphragm or piston, and this is connected to a rod which is extended to apply a braking force and retracted to release the braking force. The pressure responsive element is mounted in a chamber within the brake actuator and divides the chamber into two compartments, a first of which is closed, and thus may be pressurised, and a second of which is open to the atmosphere. The pressure responsive element is biased to a position where the rod is withdrawn by means of a return spring provided in the brake and/or a first resilient biasing element such as a helical compression spring which is located in the second compartment, and is moved to a position where the rod is extended by the supply of pneumatic fluid pressure to the first compartment.
Thus, service braking is achieved by the supply of pressurised fluid to the first compartment of the chamber, and this may be controlled by means of an ABS or EBS valve which is connected to a source of pressurised fluid such as compressed air.
To apply the parking brake, there is provided a second piston or other pressure responsive element which is biased into engagement with the first pressure responsive element by means of a second resilient biasing element, typically a helical compression spring.
The second resilient biasing element which is mounted in a second chamber within the brake actuator, and conventionally, the second chamber is open, and vents to the atmosphere. The second biasing element acts on the second pressure responsive element to move the first pressure responsive element against the biasing force of the first biasing element into a position where the rod is extended. Thus a braking force is applied, irrespective of the supply of fluid pressure to the first compartment.
In order to release the parking brake, the second pressure responsive element is moved against the biasing force of the second biasing element, in order to allow the first pressure responsive element to move under the biasing force of the first biasing element to retract the rod. This may be achieved by either mechanical means or by the application of pneumatic fluid pressure to the second pressure responsive element.
In the latter case, the brake actuator is also provided with a quick release valve which, when opened, releases any pressurised fluid acting on the second pressure responsive element to apply the parking brake. Thus, the quick release valve may be opened in order to operate the parking brake. Where the parking brake is adapted to be released by means of pneumatic fluid pressure, for example using compressed air, the quick release valve typically vents the fluid to the atmosphere.
In such conventional brake actuators, both the second compartments are exposed to air from around the brake actuator, which is drawn into the second compartment of the first chamber and the second chamber respectively, whenever the volume of the respective second compartment is allowed to expand. Such air is often damp, and may contain aggressive chemicals such as salt, and therefore may cause corrosion of the interior surfaces of the second compartment and any resilient biasing element contained therein, which ultimately may result in failure of the brake actuator.